Titanium (Ti) is widely used as an implant material in dental and orthopedic applications because of its biocompatibility, corrosion resistance, and mechanical durability. Commercial-purity, dense Ti has one of the smallest elastic modulus values (100-110 GPa) among the metallic materials commonly used as implants in the biomedical industry; however, its modulus is still far larger than that of human cortical bone (10-15 MPa). It is well documented that the use of implants with elastic modulus values far larger than that of bone can lead to undesirable bone resorption resulting from stress shielding.
Modification of the microstructure and macro-shape is a widely-used approach for controlling the mechanical properties of solids. For a given macro-shape, modification of the pore characteristics such as the porosity, pore size, and distribution in the pore size provides a method for altering the mechanical properties. Porosity is also important in implant applications; an implant should have the requisite pore characteristics to support tissue in-growth and integration with host tissues. Generally, interconnected pores of size larger than 100 μm have been reported to be beneficial for supporting bone in-growth.
Because of the importance of porosity in implant applications, the production of porous Ti has been the subject of several investigations in the last few decades. The methods include conventional powder metallurgy, solid freeform fabrication (e.g., selective electron beam melting and three-dimensional printing), sintering of powders, tape casting, and foam replication techniques. For example, a conventional powder metallurgy method involves compacting Ti particles in a die, and sintering the construct in a vacuum or inert gas atmosphere to bond the Ti particles into a strong network. However, this method provides only a limited range of porosity (approximately 30 to 50%) which makes it difficult to match the mechanical properties of bone. Another method, the polymer foam replication technique, involves the steps of (1) coating a polymer foam with Ti (or TiH2) particles, (2) decomposing the foam in a vacuum, and (3) sintering the construct in a vacuum to bond the Ti particles. This method requires decomposing a large mass of polymer foam in a high-vacuum furnace which is detrimental to a high-vacuum furnace, particularly when fabricating a large article. Organic space holder methods employ carbamide, ammonium hydrogen carbonate, or other materials as a space holder in the fabrication. The major drawback of these methods is the removal of the organic space holders which generate environmentally hazardous vapors. The rapid prototyping methods can fabricate highly controlled pore structure and pore size distribution, but they require expensive equipment.
More recently, hierarchically-structured Ti foams have been produced by first forming an oxide precursor by a gel-casting method, followed by electrochemical reduction. Current production of most reactive metals by a powder metallurgy route involves a controlled-atmosphere sintering step in which a compacted mass of particles is heated in a vacuum or in a high-purity inert gas atmosphere to bond the particles. However, the use of a vacuum furnace or an inert gas atmosphere furnace leads to high fabrication costs.
Therefore, there is a need to provide a new and improved method for fabricating porous titanium with controlled pore sizes, wide porosity range, and low-cost process.